1. Field of the Invention
This invention relates to high zirconia fused cast refractories for a glass tank furnace having an excellent corrosion resistance, particularly suitable for a glass tank furnace of low alkali glass or for an electrical glass melting furnace.
2. Discussion of Background
Fused cast refractories are provided by melting a mixed raw material of refractories which has been prepared to predetermined components in an electric arc furnace normally employing graphite electrodes, casting molten material into a heat-insulated mold having a predetermined shape, and cooling to solidify the molten material. These refractories are known as refractories having very dense structures and excellent corrosion resistances, compared with normally employed bonded refractories (for instance, brick which is fired after press-forming).
Among such fused cast refractories, especially, refractories whose major component is zirconia (ZrO.sub.2) are provided with good corrosion resistance against molten glass, and therefore, fused cast refractories having much content of ZrO.sub.2 are preferably employed for glass tank furnaces.
As a former invention of high zirconia fused cast refractories including ZrO.sub.2 of not smaller than 62 weight %, there is Japanese Examined Patent Publication No. 32408/1973.
However, fused cast refractories which have been disclosed as Examples in the specification of the Japanese Examined Patent Publication No. 32408/1973 contain ZrO.sub.2 of 88.7 weight % at the most, which indicates that there was no technology at that time to cast fused cast refractories including 90 weight % or more of ZrO.sub.2 without cracking.
Thereafter, high zirconia fused cast refractories including ZrO.sub.2 of not smaller than 90 weight % have been proposed in Japanese Examined Patent Publication No. 39090/1975 or Japanese Examined Patent Publication No. 12619/1984, and have been on sale. Thereafter, high zirconia fused cast refractories have been employed as refractories for glass tank furnaces, at portions of the glass tank furnaces wherein a particular corrosion resistance was required, since the high zirconia fused cast refractories were hard to contaminate glass matrix and their blistering behavior is insignificant. Further, in recent times, the usage of the high zirconia fused cast refractories is being widened to a field of a glass tank furnace which melts fine glass having small content of alkaline components at a high melting temperature.
These high zirconia fused cast refractories include a comparatively small amount of matrix glass whose major component is silica (SiO.sub.2). Baddeleyite (ZrO.sub.2) crystal which is a major component of these refractories, causes a volume change due to a reversible crystal phase transformation between a monoclinic crystal and a tetragonal crystal in a temperature range of 800.degree. through 1250.degree. C. which is peculiar to baddeleyite crystal. The matrix glass in the refractories is prepared to be a glass having a pertinent softness at 800.degree. through 1250.degree. C. which is the transformation temperature range of baddeleyite crystal, so as to absorb this volume change and to relax stresses caused in the refractories.
In high zirconia fused cast refractories proposed in the Japanese Examined Patent Publication No. 39090/1975 or the Japanese Examined Patent Publication No. 12619/1984, an alkaline component (Na.sub.2 O or K.sub.2 O) is added to the refractories as a component for decreasing the viscosity of the matrix glass, which provides a pertinent viscosity capable of alleviating the stresses caused in the refractories, in the transformation temperature range of baddeleyite crystal.
However, when these refractories shown in the examples are employed as lining refractories of a glass tank furnace for melting low alkali glass, the alkaline component tends to dissolve in the molten glass. Moreover, when glass is electrically melted by feeding electricity directly through the molten glass, the matrix glass containing the alkaline component, that is present in these refractories, shows an ionic conductivity in the service temperature, and a portion of the fed electricity does not flow through the molten glass but flows through the refractories surrounding the molten glass and is uselessly consumed. Therefore, these high zirconia fused cast refractories are not suitable for the refractories for the electric glass melting furnace.
Japanese Examined Patent Publication No. 40018/1990 proposes high zirconia fused cast refractories wherein the content of alkaline component of Na.sub.2 O, K.sub.2 O and the like in the refractories is restrained to not larger than 0.10 weight %, by containing much content of H.sub.2 O having a large ionic radius that is not so effective to reduce the electric resistance and is provided with a high electric resistivity at a high service temperature. However, these high zirconia fused cast refractories show "chipping off phenomenon" wherein the surface layer of the refractories is partially chipped off in heating up the glass tank furnace, and is provided with the problem of thermal cycle resistance, wherein a volume increase after each thermal cycle is accumulated finally to cause cracking, when the refractories undergo thermal cycles which traverses the phase transformation temperature range.
Japanese Unexamined Patent Publication No. 285173/1988 proposes high zirconia fused cast refractories having a large electric resistivity at a high service temperature (1500.degree. C.). These fused cast refractories are provided with a composition which does not substantially include the Na.sub.2 O component having a small ionic radius and a considerably small electric resistivity. These fused cast refractories include B.sub.2 O.sub.3 of 0.5 through 1.5 weight % and not larger than 1.5 weight % of K.sub.2 O or the like having a large ionic radius, thereby adjusting the viscosity of the matrix glass, and having a large electric resistivity and can be cast with almost no cracking.
However, according to embodiment described in the specification, no example has been shown that the refractories are provided with the thermal cycle resistance. These fused cast refractories are believed to be poor at thermal cycle resistance, since the content of SiO.sub.2 is not larger than 6.5 weight % in every case, and is silent on the presence or absence of the chipping off phenomena which causes defects in glass products and its method of solution.
In the meantime, Japanese Unexamined Patent Publication No. 100068/1989, Japanese Unexamined Patent Publication No. 218980/1991 and Japanese Unexamined Patent Publication No. 28175/1991, propose high zirconia fused cast refractories wherein the content of the alkaline component is not so much restricted and both the chipping off phenomenon and the thermal cycle resistance have been improved.
In case of the high zirconia fused cast refractories having a large electric resistivity at high service temperatures, wherein the content of the alkaline component is restrained to not larger than 0.10 weight %, the chipping off phenomenon of the refractories and the problem of causing crackings by the accumulation of volume increase after each thermal cycle, are expected yet to solve, to promote the quality and the yield rate of glass products and to provide the reliability and the durability of glass tank furnaces.